1. Field of the Invention
The present invention is in the field of spark gap switches, and particularly relates to a coaxial spark gap switch capable of switching 40 kiloamperes at 250 kilovolts at repetition rates on the order of 1,000 Hz and producing current pulses only a few nanoseconds wide, the pulses having a rise time on the order of one nanosecond.
2. The Prior Art
In U.S. Pat. No. 3,042,828 issued July 3, 1962, Josephson discloses a spark gap switch having a trigger electrode and usable at high voltages and currents. Josephson's switch is not coaxial, and there is no provision for flowing a fluid through the switch for cooling and cleaning purposes.
In the U.S. Pat. No. 3,983,438, issued Sept. 28, 1976 to Levatter et al., there is shown a spark gap switch having two electrodes in a coaxial configuration, and employing a dielectric liquid or a saturated vapor flowing through the switch. The switch disclosed in this patent does not include a trigger electrode.
The present invention grew out of attempts to solve a problem which was not addressed by the known prior art and which had long frustrated workers in the range of parameters recited above. That problem was how to obtain pulse repetition rates in the kilohertz range.
After each firing of a spark gap, a cloud of ions remains in the space between the electrodes and on the electrodes. These ions are usually swept out of the gap by a stream of dielectric fluid directed through the gap. The spark gap cannot be re-fired until the ions have been swept out, since their presence will lead to premature or even spontaneous firing. Normally it is the time required for sweeping out the ions which limits the firing rate (pulse repetition frequency) of a switch.
Although it might seem plausible to attempt to increase the firing rate by increasing the flow velocity of the sweeping fluid, this approach is fraught with serious problems which rendered it impractical at the time of the present invention.
Typically, the sweeping fluid was pressurized and the pressure drove the fluid through a constricted structure near the gap. Higher flow speeds were obtainable only be increasing the pressure, and required a stronger and heavier structure to confine the pressurized fluid.
It was also found that electrode erosion was accelerated at higher flow speeds, thereby shortening the useful life of the spark gap.
Further, it was found that to obtain the higher pressures needed to produce the increased flow speeds required prodigious amounts of pumping power. In a typical system, the pumping power was proportional to the third power (cube) of the flow speed. To obtain a three-fold increase in firing rate required a three-fold increase in flow speed of the sweeping fluid, and this, in turn, required a twenty-seven-fold increase in pumping power.
At the time of the present invention, further increases in the firing rate could not be obtained in a practical device because of these limiting factors. A fundamentally different approach was needed, and the present invention arose in response to this need.